1. Field of the Invention
The present invention relates to a toothed belt power transmission system including a toothed belt and a toothed pulley used in combination for performing power transmission in a phased power transmission mechanism between the crankshaft and camshaft of an engine, a roller conveyor mechanism of an article conveyance machine, a drive mechanism of a fabric sewing machine, and a drive mechanism of a printer.
2. Description of the Related Art
Toothed belt power transmitting systems comprised of a toothed belt and a toothed pulley, which are widely used for power transmission between a crankshaft and a camshaft of an engine, are generally classified into two groups; one using a rectilinear tooth profile having a flat engagement surface A1 on both teeth of a toothed belt and teeth of a toothed pulley, such as shown in FIG. 6A, and the other having a curvilinear tooth profile having a curved engagement surface A2 on both teeth of a toothed belt and teeth of a toothed pulley, such as shown in FIG. 6B (see Japanese Patent Publication No. Hei-5-70735).
The toothed belt power transmission system shown in FIG. 6A is able to achieve power transmission with high efficiency because a maximum contact area is available due to the flat engagement surfaces A1 of the toothed belt and pulley. However, to secure smooth meshing engagement between the toothed belt and pulley, a sufficiently large clearance or gap A3 must be provided between the tooth of the belt and the groove of the pulley. Due to the presence of such large clearance A3, the tip of a belt tooth is likely to be damaged or otherwise broken when subjected to an impact force applied at the mesh with a pulley tooth in a heavy-load high-speed operation. The broken tooth may cause considerably deterioration of the durability of the toothed belt and jumping of the pulley teeth, rendering the power transmitting operation unstable.
The toothed belt power transmission system shown in FIG. 6B can achieve smooth power transmitting operation because the respective engagement tooth surfaces A2 of the belt and pulley are curved so as to define therebetween a predetermined relatively small clearance extending from the root of a belt tooth toward the tip and terminating short of the belt tooth tip. However, at the onset of meshing engagement under heavy-load and high-speed operating condition, the curved engagement surface of a belt tooth is brought into contact with the curved engagement surface of a pulley tooth over the entire area thereof. This cause air to be trapped between the curved engagement surfaces and finally produce a bursting sound, thereby increasing the operation noise of the power transmission system. Additionally, root and tip portions of the belt tooth are susceptible to wear and cracking due to abrasion and friction heat concentrated thereto. This results in considerable reduction of the durability and wear-resistance of the toothed belt.
It is accordingly an object of the present invention to provide a toothed belt power transmission system which is capable of suppressing a shock at the mesh of a toothed belt with a toothed pulley under heavy-load and high-speed operating condition, can reduce meshing noise, and is able to improve the durability of the toothed belt.
To achieve the foregoing object, a toothed belt power transmission system according to the present invention comprises: a toothed pulley having evenly spaced pulley teeth on a peripheral surface thereof, each of the pulley teeth having a curved engagement surface on a side thereof, the curved engagement surface including a concavely arcuate root portion and a convexly arcuate tip portion located respectively adjacent to a root spaced from the outer periphery and a crest adjacent the outer periphery of the pulley tooth, the pulley teeth each having a pressure angle formed at the engagement surface; and a toothed belt having evenly spaced belt teeth on one surface thereof, each of the belt teeth having a curved engagement surface on a side thereof adapted to engage the curved engagement surface of one pulley tooth for power transmission between the toothed pulley and the toothed belt, the curved engagement surface of each belt tooth including a convexly arcuate tip portion and a concavely arcuate root portion located respectively adjacent to a crest and a root of the belt tooth, the convexly arcuate tip portion and concavely arcuate root portion of the belt tooth being slidably engageable with the concavely arcuate root portion and convexly arcuate tip portion, respectively, of the pulley tooth, the belt teeth each having a pressure angle formed at the engagement surface, the pressure angle of the belt teeth being slightly greater than the pressure angle of the pulley teeth.
At the onset of meshing engagement between one belt tooth and a mating pulley tooth, the convexly arcuate tip portion and concavely arcuate root portion of the belt tooth are in sliding contact with the concavely arcuate root portion and convexly arcuate tip portion, respectively, of the pulley tooth in such a manner that the confronting engagement surfaces of the belt tooth and pulley tooth partially separate from each other to define therebetween a space opening at opposite ends in the transverse direction of the toothed belt.
By thus formed space, air confined between the belt tooth and the pulley tooth is allowed to escape and does never produce a bursting sound. This reduces the meshing noise generated when the toothed belt meshes with the toothed pulley under heavy-load high-speed operating conditions. Additionally, since the space formed between the confronting engagement surfaces of the engaging belt and pulley teeth achieves a certain cushioning effect at the onset of meshing engagement, a meshing impact force acting on the contacting engagement surface portions of the belt and pulley teeth is pneumatically dampened. This improves the durability of the toothed belt.
As the meshing engagement advances, the rubber at the slidably contacting engagement surface portions (tip and root portions) of the belt tooth gradually deforms to follow or assume the profile of the engagement surface of the pulley tooth. This condition continues throughout the power transmitting operation between the mating pair of belt and pulley teeth. During the power transmitting operation, the engagement surface of the belt tooth elastically deformed arcuately about the intermeshing line engages the engagement surface of the pulley tooth over the entire area thereof. Thus, the belt teeth of the present invention are substantially free from damage such as a break or a crack which may otherwise occur at the convexly arcuate tip portion or the concavely arcuate root portion as in the conventional toothed belt. The toothed belt of the present invention is highly durable and is able to reduce power transmission loss during a heavy-load high-speed operation.
The crest of the belt teeth may be a flat surface or may have a dimple so as to facilitate elastic deformation of the belt tooth when the belt tooth comes in complete mesh with the mating pulley teeth. The belt tooth having such dimple contributes to further reduction of meshing noise.
In one preferred form of the invention, the convexly arcuate tip portion and concavely arcuate root portion of the belt tooth merge together without forming an angle therebetween, and the concavely arcuate root portion and convexly arcuate tip portion of the pulley tooth merge together without forming an angle therebetween. The arcuate portions may be a circular arcuate surface, a quadric surface or a parabolic surface.
It is preferable that the convexly arcuate tip portion of the pulley tooth is described by a circular arc, and the concavely arcuate root portion of the belt tooth is described by a circular arc having a radius of curvature slightly greater than that of the circular arc of the convexly arcuate tip portion of the pulley tooth.
The above and other objects, features and advantages of the present invention will become manifest to those versed in the art upon making reference to the following description and accompanying sheets of drawings in which certain preferred structural embodiments incorporating the principle of the invention are shown by way of illustrative example.